Yeast are unicellular fungi that are considered to be facultatively anaerobic. This means that in the presence of oxygen they can and will undergo aerobic respiration, but at the same time they will likely be fermenting glucose. Furthermore, when no oxygen is present they can survive, potentially forever, by fermentation. Yeast use ethanol fermentation, and this produces carbon dioxide as one of its final products.

One way of quantifying the rate of fermentation in yeast is to measure the rate at which the volume of gas changes in a test tube connected to a solution of yeast being fed with glucose. Since carbon dioxide is the only gas involved in fermentation, the rate of gas production should mirror the rate of fermentation.

The following table shows the amount of gas evolved in such an experiment under different environmental conditions.

pH

Gas evolved (in ml)

5

5

6

10

7

20

8

30

9

25

10

10

Assuming that respiration and fermentation occur at the same time, which of the following statements is true about this experiment?

Options

Since respiration yields more ATP per glucose molecule than fermentation does, the net amount of ATP produced by respiration must be greater than that produced by fermentation.
Even though respiration yields more ATP per glucose molecule than fermentation does, the net amount of ATP produced by fermentation may be greater than that produced by respiration, as long as many more glucose molecules are processed by fermentation than respiration.
Fermentation cannot take place in the presence of oxygen, because it is so inefficient.
The question is impossible to answer, because respiration and fermentation can never take place at the same time.